Sleep timing and quality change with age, with earlier bed and wake times and increased awakenings during the latter part of the night. Age-related changes in circadian rhythms are thought to underlie these changes in sleep. We have found that the intrinsic period of the circadian system in healthy older people is not shorter than in young adults, and the average amplitude of the melatonin secretion pattern is not reduced in healthy older adults when compared with young adults, eliminating potential explanations for the change in sleep timing and quality with age. However, we have found that the relationship between the timing of the habitual sleep episode and the phase of the circadian rhythms of core temperature and plasma melatonin changes with age, such that older individuals not only wake at an earlier clock hour they wake at an earlier circadian phase. We have also found that when older subjects sleep at adverse circadian phases, their sleep is more disturbed than the sleep of young adults, and there is a narrower range of circadian phases over which older subjects can maintain sleep. However, in those studies, subjects lived in the laboratory for at least 4 days before the measurements of circadian phase were completed, and it is not clear how exposure to laboratory lighting conditions affected those findings. In order to assess the phase relationship between the habitual sleep episode and the circadian timing system in older people living in society, and in order to assess the impact of that phase relationship on sleep quality, 4 testable hypotheses are proposed: (1) the phase of the plasma melatonin rhythm, measured immediately upon entry into laboratory conditions, occurs at an earlier hour in healthy older people; (2) the phase relationship between the circadian rhythm of melatonin and the timing of habitual awakening is such that older adults wake at an earlier hour and an earlier circadian phase; (3) the average light exposure of older subjects is less than that of young adults; and (4) sleep efficiency is greater in those older subjects whose melatonin phase occurs near the end of sleep than in those subjects whose phase occurs near the beginning of sleep. These results will provide preliminary data for understanding how sleep timing and circadian rhythmicity interact in older people living in society, and how light exposure influences those interactions.